1. Field of the Invention
The present invention relates to a new and improved registration system for light exposure units of the type utilized for exposing photosensitive sheets to controlled light through image forming film sheets for reproducing images. More particularly, the present invention is especially useful in multi-color copy making operations wherein a plurality of different photosensitive sheets and image forming film sheets are needed and wherein precise registration is required in order to accurately reproduce precision color images in three and four color printing operations.
2. Background of the Prior Art
In copy making and platemaking operations where precise registration is required between one or more image forming films and presensitive sheets, especially in three and four color operations, fast draw-down type vacuum blankets have been utilized. One such vacuum blanket system is shown and described in copending U.S. patent application Ser. No. 353,055, filed May 17, 1989, assigned to the same assignee as the present application and incorporated herein by reference. Because these vacuum blankets operate by using nonuniform clamping force distribution to induce air flow channels in the film media, the forced distribution of air propagates along thin, sheet-like inserts commonly known as a "Carlson" boards which are inserted between the vacuum blanket and the film sheets. With a relatively thin, stiff, "Carlson" board, the fast draw-down vacuum blanket is extremely effective. However, when relatively thick "Carlson" boards are utilized, collapsible registration pins are needed and because the "Carlson" board is relatively stiff and thick, and the fast draw-down action may be lost. Registration pins are required to provide precise registration between successive or multiple image forming films and one or more photosensitive media sheets on which images are to be reproduced. As wear occurs in a "Carlson" board, the registration pins begin to fit loosely in the registration holes provided in the Carlson board and the pins tend to fall out during handling and copy making operations. This problem causes delays in production and in addition, results in imprecise registration because the worn pins may begin to tilt or move slightly as the relatively thin supporting surface around the registration opening in a "Carlson" board becomes worn. The tilting or movement of a registration pin results in imprecise registration between successive image forming films and presensitized sheets.